Test for non-reducing sugars after hydrolysis
Acid hydrolysis breaks non-reducing sugars into reducing sugars that can then be detected with Benedict solution.
Section 2.1 · Testing for biological molecules
AS Level · Cambridge International Biology 9700, 2028–2030. Follow school laboratory risk assessments, reagent instructions and supervised practical procedures. Written practice is self-checked and cannot confirm practical competence.
Before you begin
- Recognise that biological samples contain molecules that can be tested with suitable reagents.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why is dilute acid used in the non-reducing sugar test?
What you will learn
- Describe a test for non-reducing sugars using acid hydrolysis and Benedict solution
- Explain why neutralisation and a before-hydrolysis comparison matter
Hydrolyse before the second Benedict test
First test a portion of the sample with Benedict solution and heat it in a water bath. If reducing sugar is detected, record that result before testing for non-reducing sugar; this provides a comparison.
For a separate portion, add dilute acid and heat to hydrolyse the non-reducing sugar. Hydrolysis breaks glycosidic bonds and may release reducing monosaccharides. Cool the sample and neutralise the acid using the approved laboratory procedure before adding Benedict solution.
Interpret the change carefully
After neutralisation, add Benedict solution and heat in a water bath. A positive result after hydrolysis, when reducing sugar was not detected in the untreated portion, supports the presence of non-reducing sugar. The comparison distinguishes sugar created by hydrolysis from reducing sugar already present.
Acid can interfere with Benedict solution, so neutralisation is important. Follow the practical method and safety instructions for dilute acid and heating. A positive result identifies reducing products after hydrolysis; it is not a direct measurement of the original non-reducing sugar concentration.
Pause and explain
What should happen before adding Benedict solution to the hydrolysed sample?
Worked example
An untreated sample gives no Benedict colour change. A matched portion is acid-hydrolysed, cooled, neutralised and then gives an orange precipitate with Benedict solution. Explain the evidence.
Show the worked solution
- Untreated result did not detect reducing sugar.
- Hydrolysis can break non-reducing sugar into reducing products.
- The post-hydrolysis Benedict result is positive.
Answer The untreated portion gave no detectable reducing-sugar result, while the hydrolysed and neutralised portion gave a positive result. This supports the presence of a non-reducing sugar that was hydrolysed to reducing sugars. The test does not directly quantify the original non-reducing sugar.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the main stages used to test for a non-reducing sugar.
Give me a hint
Keep untreated and hydrolysed portions distinct.
Compare my reasoning
- Test an untreated portion with Benedict solution first.
- Hydrolyse a separate portion with dilute acid, cool, neutralise, then add Benedict and heat.
Test one portion with Benedict solution and heat to check for reducing sugars already present. For a separate portion, add dilute acid and heat to hydrolyse the sample, cool and neutralise it with the approved method, then add Benedict solution and heat in a water bath. Compare the results.
Look for these in your work
- Before-hydrolysis test included
- Hydrolysis, neutralisation and final Benedict test in order
A sample is negative before hydrolysis and positive after it. What conclusion is supported?
Give me a hint
Compare the two tests.
Compare my reasoning
- Hydrolysis produced detectable reducing sugar.
- This supports a non-reducing sugar in the original sample.
The change from no detected reducing sugar before hydrolysis to a positive Benedict result after hydrolysis supports the presence of a non-reducing sugar that was converted to reducing products.
Look for these in your work
- Result change described
- Conclusion appropriately cautious
A student boils an acid-hydrolysed sample with Benedict solution without neutralising it. Explain the procedural flaw.
Give me a hint
Consider Benedict solution’s required conditions.
Compare my reasoning
- The sample remains acidic.
- Neutralise after cooling before adding Benedict solution.
The sample is still acidic, which can interfere with Benedict solution. Cool and neutralise the hydrolysate using the approved method before adding Benedict solution and heating it.
Look for these in your work
- Acidity issue identified
- Safe corrected order
Common mistakes
- Adding Benedict solution before neutralising the acid
- Skipping the untreated comparison
- Interpreting the post-hydrolysis precipitate as the original sugar’s direct result
What can you explain now?
Why is dilute acid used in the non-reducing sugar test?
Compare with the explanation
To hydrolyse the sugar into reducing products
Hydrolysis breaks bonds in the sugar.
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Make it stick
Why compare a sample before and after acid hydrolysis when testing for non-reducing sugar?
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